The invention relates generally to side scan sonar systems for mapping the topography of an underwater seabed and in particular, to electrical amplifiers for reducing the dynamic range of the side scan sonar return signals.
Side scanning sonar systems are well known in the art. They typically comprise a transmitting transducer for emitting a substantially planar acoustical signal and a receiving transducer for providing an electrical output signal representing the returning acoustical signals. The receiving transducer output is electrically amplified and processed and the processed electrical signal is applied to a graphics printer for providing a permanent record of the seabed topography. Typically, due to attenuation as a result of beam spreading, absorption, and other time dependent effects which occur as the acoustical signal travels through the fluid medium, the dynamic range of the incoming signal is very large, typically being on the order of 120 decibels. As a result, the large dynamic range of the input signal is reduced to provide electrical signals representing the acoustical return signals which have a dynamic range more closely adaptable to the output display apparatus.
The resulting time varying gain amplifiers, generally referred to as TVG amplifiers, have a gain which varies as a function of time. Typically, according to the prior art, these amplifiers are set by switches positioned on a front panel of the sonar control device. Thus, as the result of the sonar scan being recorded, a skilled technician or operator will vary the parameters affecting the gain and contrast to provide a more "aesthetically pleasing" output record, that is, a record which corresponds to that which a skilled operator expects to see.
Unfortunately, several difficulties arise if the seabed topography is different than that which is expected. Thus, the output record may depict only the operator's expectations and not the actual seabed topography. In addition, manual manipulation of the controls for the time varying gain amplifier inserts, into an already sensitive system, variable parameters whose values are generally not recorded on the output record. Thus, when a skilled operator or interpreter views the record at a later date, he may have no knowledge of the conditions under which the record was made.
The principal objects of this invention are therefore a time varying gain amplifier which has a predetermined gain as a function of time; and a time varying gain amplifier which is reliable, self-contained, and which has an output gain versus time which is representative of the sonar signal attenuation as it traverses a fluid medium.